Isotopic characterization of Brazilian ketchup: Is tomato its main ingredient?

Ketchup is a popular tomato-based sauce also containing differing proportions of sugar and vinegar. While tomato is a plant with the C3 photosynthetic metabolism, sugarcane is a tropical grass with C4 photosynthesis, used in Brazil as feedstock for sugar and alcohol vinegar production. Sugarcane derivatives are often widely used in the Brazilian food industry. In this paper, we investigated the relative isotopic proportion of C3 and C4-derived products in 25 ketchup brands commercialized in Brazil, through the carbon (C) isotopic ratio (delta 13C). The aim was to identify the contribution of tomatoes to the final ketchup composition, as the manufacturers do not provide this information to the consumer. The overall delta 13C mean in the analyzed ketchup was -16.2 +/- 1.4 parts per thousand, suggesting a predominance of C4- over C3-derived sources in these brands. Using a Bayesian isotopic dilution model, we estimated that approximately 75 % of the carbon source of ketchup comes from C4 plants (p < 0.05). A low correlation between ketchup price and delta 13C was found, probably because price reflects attributes such as brand and, not necessarily, product composition. In addition, our study also presents a robust regressive predictive model of the sugar percentages in the final ketchup composition. Combining these approaches, consistent tools can be incorporated to evaluate ketchup quality.